1. Field of the Invention
The present invention relates to an actuator, and particularly relates to an actuator that is installed in a digital camera, mobile phone, or other compact precision apparatus and drives a zoom lens.
2. Description of the Invention
As a driving device for a lens unit of a digital camera, etc., an actuator that employs a piezoelectric element is used. For example, with an actuator of Japanese Patent No. 2633066, a driving shaft is affixed to one side of a piezoelectric element, and the other side of the piezoelectric element is fixed to a main device body. A lens barrel is slidably supported on the driving shaft, and the lens barrel is frictionally engaged with the driving shaft by making use of an urging force of a plate spring. Drive pulses of substantially sawtooth-like waveform are applied to the piezoelectric element, and the piezoelectric element deforms at different speeds in an extension direction and a contraction direction. For example, when the piezoelectric element gradually deforms, the lens barrel moves along with the driving shaft. Oppositely, when the piezoelectric element rapidly deforms, the lens barrel stays at the same position due to its mass inertia. Thus, by repeatedly applying drive pulses of substantially sawtooth-like waveform to the piezoelectric element, the lens barrel can be moved intermittently at a fine pitch.
However, with the actuator of Japanese Patent No. 2633066, the piezoelectric element frequently undergoes resonant vibration, and because, the piezoelectric element or the driving shaft undergoes lateral vibration or torsion in directions that differ from the displacement direction due to this resonant vibration, a driven member does not accurately move in the displacement direction. The actuator of Japanese Patent No. 2633066 is thus unstable in driving amount with respect to driving frequency.
As a method for resolving the above issue, a method for displaceably supporting the other side of the piezoelectric element and attaching a weight member to the other side of the piezoelectric element may be considered.
However, with this method, because when the weight of the weight member is small, the moving distance, moving speed, and thrust of the driven member are inadequate, a weight member of large weight must be used and the device thus becomes large. Especially when a driven member is to be moved across a large moving distance as in the case of zoom lens, a weight member of extremely large weight is necessary and the device becomes large.